The overall performance of an engine in terms of combustion efficiency, speed control, exhaust emission of pollutants and others, greatly depends on controlling the mixing of the air and fuel into an appropriate ratio for combustion and on regulating the flow of this mixture into the combustion part of the engine. Precise and reliable control of the combustion is very important for the efficiency and the safety of the combustion process, as is well understood by those skilled in the art. For example, it is well known that combusting a fuel with excess oxidant yields higher nitrogen oxides (NOx) emission rates. Combustion of a fuel with an uncontrolled excess amount of air can also lead to excessive fuel consumption and increase the production cost of the final product. On the other hand, incomplete combustion of a fuel generates carbon monoxide (CO).
NOx, CO and hydrocarbon (HC) emissions are regulated by the government to increasingly lower levels and in an ever increasing number of industries. In addition to the NOx, CO and HC emissions, many designs must meet the requirements of regulatory agencies that have adopted the standards published by governments, insurers, and industry organizations (such as UL, CSA, FMRC, etc.). These concerns relative to fuel supplies and air quality have led government agencies to reducing the amount of allowed emissions in gaseous fueled engines in an effort to reduce pollution in the environment. More restrictive emissions limits are being issued by governments. For example, emissions limits have been set for the first time in 2004 on engines used in forklifts and other nonroad vehicles such as airport ground-service equipment, off-highway motorcycles, all-terrain vehicles, snowmobiles, and recreational marine diesel engines. The rule will require that nitrogen oxides (NOx) emissions reductions from large industrial spark engines be implemented in two stages. Initial reductions are required in 2004 with greater reductions required in 2007.
One approach to aid in meeting the emission requirements is to use cleaner burning fuels. For example, propane is clean burning, non toxic, and produces fewer greenhouse gases (carbon monoxide, hydrocarbons and NOx) than gasoline or diesel fuel. In systems using propane, it is critical to control the flow of air and gaseous fuel such that an optimum fuel-air mixture is maintained at all times during operation of the engines. Normal operating conditions for spark ignited engines such as forklifts and other vehicles include fast transients on speed and power output, pronounced swings in air and/or fuel temperatures, and variations in fuel composition, all of which increase the difficulty of maintaining fuel-air mixtures within desired tolerances.
Mixers are used to meter the gas flow rate according to the air flow rate such that the two flow rates are maintained in proportion as the flow rates change. There are essentially two known types of mixers designed to achieve proportioned flow rates. The first type utilizes a pressure differential across an orifice, or a nozzle, in the air stream to induce gas flow, or to control an orifice size in the gas stream. The second type of mixer involves measurement of air flow rate, determining the required gas flow, and controlling a fuel injector to deliver the desired fuel flow.
Mixers of the first type employ pressure differential in the air stream to control gas flow and achieve the objective of maintaining a constant fuel-air ratio in a number of ways. For example, the air-valve type, used for many years in industrial gas engines, applies a pressure differential generated by air flow on a diaphragm to modulate a gas valve. Disadvantages to this type of mixer are the diaphragm is always in motion when engine power is varied continuously, causing wear and consequently affecting the reliability of the mixer. Additionally, the fuel-air flow ratio is affected by a number of operational parameters, making it difficult to obtain a constant fuel-air ratio over a wide range of engine power. Such disadvantages have not been important factors in industrial gas engines which operate at steady power for a majority of the time, but have a pronounced detrimental effect on emissions when applied to vehicular engines.
Another mixer of the first type is the venturi mixer, which utilizes Bernoulli's law to induce gas flow into the throat of a venturi in which there is air flow. The rate of gas flow is proportional to the rate of air flow and, in theory, remains so as long as the supply pressures of air and gas are equal and the temperatures of the air and gas remain constant. A venturi mixer insures maximum performance, through mixing of the air and gas, and proportioning of the air and gas within limits. The air is generally at atmospheric pressure, which requires the fuel flow to also be at atmospheric pressure for the venturi mixer to work. Typically, a zero pressure fuel regulator (i.e., a zero governor) cancels variations in the gas line pressure and reduces it to atmospheric, thus allowing the venturi mixer to entrain gas in constant ratio to the amount of air passing through. The zero governor works on the suction principal: air flowing through a venturi mixer creates suction in the gas line that opens the zero governor, thereby allowing gas to flow.
Due to a variety of technical and safety-related issues, it is difficult, in practice, to provide a zero pressure fuel delivery to the venturi mixer for applications such as mobile applications. For example, most propane-fueled, mobile industrial equipment in use today utilizes a negative pressure vaporizer/regulator for final fuel delivery pressure control. This device typically regulates the liquid propane withdrawn from the storage tank down to a final output pressure in the range of −0.5 to −1.5 inches of water relative to a reference pressure port. The negative pressure output only slightly affects the mechanical air/fuel ratio control behavior of an air-valve mixer. However, the use of venturi mixer is usually impractical in systems with a negative pressure regulator output.